A CaO enhanced rubberized syntactic foam

Abstract

Abstract In this study, a calcium oxide (CaO) enhanced rubberized syntactic foam was developed by incorporating CaO powder into a rubber latex/microballoon mixture, in order to effectively and efficiently remove the water contained in the rubber latex in a controlled manner. It was expected that an exothermic reaction between CaO and water would be generated, which would result in the removal of water from the mixture and would leave behind glass microballoons coated by a calcium hydroxide (Ca(OH) 2 ) reinforced rubber layer. In order to validate this idea, a new manufacturing procedure was proposed. Low velocity impact and four-point bending tests were performed on pure foam and the foam cored sandwich beams. Scanning electron microscopy was used to examine the existence and uniformity of the rubber coating and X-ray diffractometry was employed to validate the presence of Ca(OH) 2 . Compared with the previous results without CaO [Li G, Jones N. Development of rubberized syntactic foam. Compos Pt A: Appl Sci Manuf 2007;38:1483–92], the current foam showed a much higher residual strength with a slight reduction in impact tolerance. The SEM observation showed a uniform rubber coating on the microballoons and the X-ray diffractometry confirmed the existence of Ca(OH) 2 crystal in the rubber coating layer. In order to reveal the underlying principle for the enhancement, a micromechanical model based on the equivalent medium theorem was proposed and a finite element analysis using ANSYS was conducted. It was found that the rubber coating on the microballoons significantly reduced the stress concentration and improved the stress distribution; the presence of the Ca(OH) 2 crystals in the rubber layer further reduced the stress concentration. A compressive stress zone ahead of or in the vicinity of the crack tip in the rubberized syntactic foam helps in blunting and arresting the microcrack from propagating into macrocrack.